It is known to use a light modulation member such as an electrochromic display device (ECD) in a vehicle combination meter. The ECD has variable light transmissivity controlled by changing voltage. The ECD is arranged in front of a display board.
According to a light transmissivity control operation of Japanese Patent No. 2848512, for example, when the ignition switch is OFF, a dc voltage with a predetermined amplitude is applied to the ECD to reduce the light transmissivity, to thereby shutter the display board. On the contrary, when the ignition switch is at ON, zero voltage or a dc voltage with an amplitude smaller than the predetermined amplitude is applied to the ECD to increase the light transmissivity, so a driver can see the display board.
Also, it is proposed to form the display board of the combination meter from a metal plate, which is gloss finished or mirror finished. In this case, however, an external light, e.g., sunlight, is likely to reflect on the metal display board and causing the driver to be blind. To restrict the reflection of the external light, it is proposed to control the light transmissivity of the ECD according to an illumination intensity of the display board when the ignition switch is at ON.
For example, when the illumination intensity of the surface of the display board is relatively high, the light transmissivity is reduced by applying dc voltage having small amplitude, to thereby restrict entry of the external light to the display board. When the illumination intensity of the surface of the display board is relatively low, the light transmissivity is increased by applying dc voltage having relatively large amplitude, thereby to maintain a visibility of the display board.
If the dc voltage is continuously applied to the ECD, the ECD is likely to be deteriorated early, and it is difficult to maintain the long life. If the ECD is deteriorated, the color of the ECD is likely to be changed from the periphery of the display area. When a metallic substance in an oxidation color layer, which partly constitutes an EC layer of the ECD, is reduced and precipitated, brown spots occur on the EC layer. This is likely to relate to a time and a value of voltage applied thereon. The peripheral color change is likely to occur near an electrode where an electric field is high.
To increase the life of the ECD, it is proposed to apply a rectangular wave alternating voltage. This method is so-called a duty drive, and a time to apply a voltage in a predetermined direction is reduced. Namely, the voltage is applied alternately in opposite directions. When the voltage is applied in one direction, the ECD changes to a colored condition having low light transmissivity. On the contrary, when the voltage is applied in a reverse direction, the ECD changes to a colorless condition having high light transmissivity. When the duty ratio is more than 50%, the colored conditions continues longer than the colorless condition, so the ECD generally maintains the colored condition. However, since the voltage is applied alternately in opposite directions, power consumption is likely to increase due to rush currents caused on applying the voltage in the respective directions.